Which of the following extensions suggest that the file is a backup copy?

The correct answer is A. .bak.

A .bak file is a backup copy of a file. It is often used to create a copy of a file before making changes to it, in case the changes are not what you want. .bak files are often hidden by default, so you may need to change your file explorer settings to see them.

The other options are not typically used for backup files. .bas files are Visual Basic source code files, .com files are executable files, and .txt files are text files.